Formaldehyde, also known as methanal or methyl aldehyde, is an industrially important chemical, used predominantly in the manufacture of resins, such as urea formaldehyde resin and melamine resin, for example. Formaldehyde resins are used as adhesives and in the production of paints and wallpapers.
During the lifetime of a product that incorporates a formaldehyde resin, the resin may break down to release formaldehyde gas into the surrounding environment. Where the products are installed or used within a confined space such as those found within domestic residences or offices, or where the products are stored and/or transported in shipping containers, the resulting concentration of formaldehyde can exceed safe limits. Formaldehyde gas is toxic, allergenic and carcinogenic and can be dangerous at concentrations as low as 100 parts-per-billion (ppb). For example, the World Health Organisation (WHO) guideline level for prolonged formaldehyde exposure is 80 ppb, and this guideline has been adopted by many countries.
Therefore, it is essential to be able to detect the presence of formaldehyde in confined spaces such as those found in buildings or containers before the concentration of formaldehyde reaches dangerous levels.
Gas sensors known in the art to detect formaldehyde include gas chromatography and optical absorption spectroscopy.
Gas chromatography mixes a gas sample with a carrier gas (such as helium or nitrogen) and which is then passed through a long cylindrical column filled with material that the gas sample must migrate through. The components of the gas sample are separated out by the time it takes each component to reach the detector. Gas chromatography can be highly accurate and reliable. However, samples take a long time to pass through the column (several hours) and must be injected into the column, making it impossible to use for continuous real-time measurements.
Optical absorption spectroscopy techniques can be employed to detect formaldehyde by measuring the absorption of light in the specific range of wavelengths absorbed by formaldehyde (250 nm to 360 nm). For example, differential optical absorption spectroscopy uses high resolution spectrometers and differential post-measurement analysis to identify narrow band features within absorption spectra. However, complex processing of the spectral data is required to separate out the target analyte signal (formaldehyde in this case) from the background of other trace chemicals that absorb light in the same region of the spectrum. In addition, high resolution spectrometers are expensive and make any gas sensor using this technique correspondingly expensive.
A further complication for the measurement of formaldehyde within enclosed environments such as those found within a building is the presence of other gaseous species that also absorb light within the same region of the spectrum as formaldehyde. For example, decanal, hexanal, acetaldehyde, ozone, nitrogen dioxide and sulphur dioxide all absorb light within the range of 240 nm to 360 nm, which encompasses the absorption band for formaldehyde.
Therefore, it is particularly difficult and impractical to measure the concentration of formaldehyde in enclosed environments such as those found within buildings using standard optical absorption techniques due to the presence of the wide variety of species present that absorb light within a similar range of wavelengths as formaldehyde.
Therefore, one aim of the present invention is to provide an affordable gas sensor capable of real-time continuous measurement of formaldehyde concentrations in enclosed environments such as those found within buildings.
Typical optical absorption gas sensors use two detectors to provide a measurement signal and a calibration signal that compensates for any variation in performance of the light source. For example, it is well known for gas sensors to use a single light source, to split the light that has passed through a sample between two detectors using a beam splitter and to then select a different wavelength or band of wavelengths for each detector using a filter. However, each optical component adds to the expense of the gas sensor and to the attenuation of light that reaches each sensor, leading to a loss of sensitivity.
Accordingly, a further aim of the invention is to provide an improved gas sensor that is cheap and has a high sensitivity to a target analyte such as formaldehyde.
Furthermore, the optical components of gas sensors are very sensitive to the wavelength of light used. When the temperature of these optical components changes, their wavelength characteristics also change. For example, the range of wavelengths that are transmitted by a filter component such as a band-pass filter, for example, may vary with temperature.
The invention further aims to provide a method of calibrating a gas sensor to account for thermal fluctuations.